Emission from regioisomeric bis(phenylethynyl)benzenes during pulse radiolysis.

Emission from charge recombination between radical cations and anions of a series of regioisomeric 1,4-, 1,3-, and 1,2-bis(phenylethynyl)benzenes (bPEBs) substituted by various electron donor and/or acceptor groups was measured during pulse radiolysis in benzene (Bz). The formation of bPEB in the excited singlet state ((1)bPEB*) can be attributed to the charge recombination between bPEB(+) and bPEB(-), which are initially generated from the radiolytic reaction. This mechanism is reasonably explained by the relationship between the annihilation enthalpy change (-DeltaH(o)) for the charge recombination of bPEB(+) and bPEB(-) and excitation energy of (1)bPEB*. Since the degree of the pi-conjugation in the S(1) state and HOMO-LUMO levels of bPEB change with the substitution pattern of phenylacetylene groups on the central benzene ring and the various kinds of donor and/or acceptor group, the fine-tuning of the emission color and intensity of bPEB can be easily carried out during pulse radiolysis in Bz. For donor-acceptor-substituted bPEB, it was found that the difference in the charge transfer conjugated pathways between donor and acceptor substituents (linear-, cross-, and "bent"-conjugated pathways) strongly influenced the HOMO-LUMO energy gap.